Coded OFDM is a technique used in communication systems to efficiently transmit high rate signals in fading channels. Due to the wide bandwidth of these signals they would normally suffer from severe frequency selective fading. This is avoided in an OFDM system by transforming the signal into a number of orthogonal components, each of these OFDM components having a bandwidth less than the coherence bandwidth of the transmission channel. By modulating these OFDM signal components onto different subcarriers, the transmission in each individual subcarrier experiences only frequency flat fading. The Forward Error Correction (FEC) coding to transmitted information streams is thus employed to further combat the fading on OFDM subcarriers.
In a COFDM receiver system, coherent detection is necessary to provide the subsequent channel decoder (usually a Viterbi decoder) with the properly demodulated constellation signals. Coherent OFDM detection requires channel estimation and tracking. In this case, the frequency-domain estimate of transmission channel, commonly termed as Channel State Information (CSI), is often used. Although most of the related research and development has to date focused on searching for accurate and robust CSI estimation methods, the incorporation of CSI into the decoding process for enhancing the channel decoder's error correction performance has also been explored and is described in the following publications:    (1) M. R. G. Butler, S. Armour, P. N. Fletcher, A. R. Nix, and D. R. Bull, “Viterbi decoding strategies for 5 GHz wireless LAN systems,” published in Proc. IEEE 54th Veh. Technol. Conf, VTC 2001 Fall, pp. 77-81.    (2) H. Sari, G. Karam, and I. Jeanclaude, “Transmission techniques for difital terrestrial TV broadcasting,” published in IEEE Communications. Magzine, vol. 83, no.2, pp. 100-109, February 1995.    (3) W. Lee, H. Park, and Park J., “Viterbi decoding method using channel state information in COFDM system,” published in IEEE Transactions on Consumer Electronics, vol. 45, no. 3, pp. 533-537, August 1999.
In the publication by M. R. G. Butler, S. Armour, P. N. Fletcher, A. R. Nix, and D. R. Bull, entitled “Viterbi decoding strategies for 5 GHz wireless LAN systems,” published in Proc. IEEE 54th Veh. Technol. Conf, VTC 2001 Fall, pp. 77-81, this technique is referred to as “soft CSI decision decoding” and has proved to be of great value in practice when the M-PSK (M-ary phase-shift keying) modulation or M-QAM (Quadrature amplitude modulation) is used for constellation mapping.
However, in a COFDM based ultra wide-band (UWB) system such as that proposed by the WiMedia Alliance to provide very high-rate wireless transmission, in addition to QPSK (Quadrature phase-shift keying) modulation, a so-called Dual-carrier modulation (DCM) scheme was proposed for constellation mapping to achieve a degree of intra-OFDM-symbol frequency diversity. This is described, for example in the WiMedia Alliance publication of D. Leeper, “Overview of MB-OFDM,” published in the website http://www.wimedia.org/, July 2005. However, in such a system, the conventional method such as that described in the publication by M. R. G. Butler, S. Armour, P. N. Fletcher, A. R. Nix, and D. R. Bull, entitled “Viterbi decoding strategies for 5 GHz wireless LAN systems,” published in Proc. IEEE 54th Veh. Technol. Conf, VTC 2001 Fall, pp. 77-81 cannot be directly applied as it is tailored for single subcarrier modulation.
It is commonly known that, to achieve lower error probability, the soft-decision, instead of a hard-decision, should be used for the Viterbi decoding. It can be shown that the difference between the performance of standard soft- and hard-decision decoding is roughly 2 dB for an AWGN channel. This is described in the publication by J. G. Proakis and M. Salehi, Communication Systems Engineering. 2nd Edition, Prentice-Hall, New Jersey, 2002. However, in practice, when the fading effect is taken into consideration in a wireless OFDM system, standard soft-decision decoding performs poorly and its performance may be even much worse than that of hard-decision decoding. This is described in, for example, the publication by M. R. G. Butler, S. Armour, P. N. Fletcher, A. R. Nix, and D. R. Bull, entitled “Viterbi decoding strategies for 5 GHz wireless LAN systems,” published in Proc. IEEE 54th Veh. Technol. Conf, VTC 2001 Fall, pp. 77-81. The significant performance degradation with standard soft-decision decoding may be due to the well-known noise amplifying effect of the frequency domain equalization process, that is, the noise on highly attenuated subcarriers is enhanced significantly when the received symbol magnitudes are normalized. As shown in the publication by M. R. G. Butler, S. Armour, P. N. Fletcher, A. R. Nix, and D. R. Bull, entitled “Viterbi decoding strategies for 5 GHz wireless LAN systems,” published in Proc. IEEE 54th Veh. Technol. Conf, VTC 2001 Fall, pp. 77-81, this unexpected performance loss can be recovered by weighting the path metrics of Viterbi decoder using the magnitude of CSI. Or, equivalently, one may simply use the CSI, {H(k)}, to weight the complex input of the modulation demapper.
Furthermore, as is also shown in the publication by M. R. G. Butler, S. Armour, P. N. Fletcher, A. R. Nix, and D. R. Bull, entitled “Viterbi decoding strategies for 5 GHz wireless LAN systems,” published in Proc. IEEE 54th Veh. Technol. Conf, VTC 2001 Fall, pp. 77-81, when the subcarriers in an OFDM system are M-PSK or M-QAM modulated, the weighting method has proved to be effective for enhancing the error correction capability of Viterbi decoder. Importantly, this implies that one constellation point is only related to a single subcarrier. In the OFDM UWB system, this is also the case for the lower rate transmission where the conventional QPSK constellation mapping/demapping is employed. However, in the case of high data rate transmission, where the DCM is involved, this method turns out to be unsuitable as one constellation point is now related to two different subcarriers.
Thus there is a need for a system and method for demapping DCM signals with improved decoding performance.